Cmdr. John Doney of the U.S. Third Fleet said sonar is essential for national defense. He pointed to what he described as an emerging threat: the quiet diesel-electric submarine. Of the 500 subs in existence worldwide, 350 are the quiet diesel-electric variety — and the tally is growing.

“They are quieter, they have more endurance. And the diesel-electrics can operate near the coast and in choke points like straits,” Doney said. “Active sonar becomes the most effective tool to locate and track those diesel submarines and the threat they represent.”

He noted that the South Korean warship Cheonan was torpedoed and sunk in 2010 by a North Korean mini-submarine.

“I would submit that the threat is real,” Doney said.

One sticking point for the Coastal Commission on Friday was that the Navy didn’t want to adopt measures suggested by the commission’s staff. The Navy has taken that stance in the past as well.

The commission staff proposed:

•Establishing larger shutdown areas, of up to 2 kilometers, when marine mammals or sea turtles are detected

•Avoiding use of sonar in sensitive areas, including marine sanctuaries and protected areas; zones with blue, fin and gray whales; and shore areas with coastal bottlenose dolphins.

•Improving the training for lookouts, who are tasked with spotting marine mammals during Navy exercises.

•Forming a contingency plan for use of near-shore explosives, if further animal deaths occur.

The last condition refers to the March 2011 incident when at least three long-beaked common dolphins were killed by an explosives drill between a half-mile and 1 mile off the Silver Strand.

Stone, of the U.S. Pacific Fleet, said the Coastal Commission’s conditions aren’t needed because the Navy already uses safeguards such as posting lookouts and using passive sonar to detect large sea creatures.

“These really do work. They are a part of the culture of the Navy. You can go aboard any ship and you will be immediately impressed by how seriously the mitigation measures are taken,” Stone said.

He added that the Navy frowns on the idea of avoiding large sections of water, such as marine sanctuaries, because it cuts down on the realism of training.

As many as 200 people attended the commission hearing, including representatives of Greenpeace, the Sierra Club, the Surfrider Foundation and the Audubon Society.